This invention relates generally to digital communications, and more specifically to clock buffers.
For many applications in integrated circuit (IC) design, the highest frequency operation for a digital logic design can generally be achieved by using a differential current steering logic family, such as bipolar emitter-coupled logic (ECL), GaAs source-coupled field-effect transistor logic (SCFL) or CMOS current mode logic (CML). While these topologies generally have inferior speed/power ratios compared to alternative logic families such as direct-coupled field-effect transistor logic (DCFL) or static CMOS, they generally operate at a two to four times higher maximum clock frequency.
For applications requiring use of these high-speed logic families, the maximum frequency of operation will eventually be limited by the ability to maintain adequate gain on the clock path through the circuit. This is due to the fact that the clock bandwidth requirement is generally twice that of data, and the clock signal will generally have to pass through several levels of buffering. Once the frequency of operation is increased beyond the unity gain bandwidth of the clock buffers, the clock signal will be attenuated through each stage of buffering, and the circuit will fail to function.
Conventional techniques for extending the bandwidth of integrated circuit clock buffers, however, suffer from various shortcomings. For example, in an approach commonly referred to as inductive peaking, an on-chip spiral inductor is added in series with a resistive load device of a differential amplifier. The inductor is sized so that at the specified operating frequency of the circuit, the reactance of the inductor partially cancels the reactance of the parasitic capacitance at the output of the differential pair. However, inductive peaking suffers from undesirable characteristics, including for example that it has a frequency dependent gain which may present a problem for low speed functionality. It also has a frequency dependent delay, particularly near the resonant frequency of the peaking. This can present a problem if the delay of the clock signal needs to be controlled with respect to other delays in the circuit. Also, the area of on-chip spiral inductors is generally quite large, on the order of a factor of ten times, as compared to the area of a typical logic gate. Finally, a circuit with inductive peaking is of limited use in buffering an arbitrary data signal with unknown frequency components.
Another conventional approach in CMOS technology utilizes multiple phases of a lower frequency clock signal. Multiple circuits can then be operated in parallel, each running off of a different phase of the clock. This type of circuit can be thought of as emulating a circuit with a virtual clock of frequency fv=fc*M, where fc is the frequency of the multiphase clock, and M is the number of phases that are used in parallel. This second approach suffers from mismatches between each of the phases of the low speed clock. The consequence is an equivalent jitter on the xe2x80x9cvirtualxe2x80x9d high-speed clock. Additionally, even if the potential problem of the clock needing to run at a frequency above the unity gain bandwidth of the clock buffers can be eliminated, a lack of sufficient bandwidth will still cause increased deterministic jitter on high-speed data signals. From a signal integrity point of view, this technique is inferior to a design in which the circuits have enough bandwidth to run off a single clock signal.
What is needed therefore is an apparatus and method for providing a high bandwidth clock buffer, which can significantly increase the maximum frequency at which CMOS technology can be used to perform high-speed logic functions.
The present invention provides, in a first aspect, a buffer including a differential amplifier for generating first and second amplified output signals in response to first and second differential amplifier input signals, a voltage follower comprising first and second voltage follower transistors for generating first and second buffer output signals in response to the first and second amplified output signals from the differential amplifier, and a steering circuit for steering current from current sources through whichever of the first and second voltage follower transistors is being pulled low to increase the amount of current.
In another aspect, the present invention provides a method for buffering signals, including the steps of generating first and second amplified output signals using a differential amplifier in response to first and second differential amplifier input signals, generating first and second buffer output signals in response to the first and second amplified output signals from the differential amplifier using a voltage follower comprising first and second voltage follower transistors, and steering current from current sources through whichever of the first and second voltage follower transistors is being pulled low to increase the amount of current that is available.
These and other features and advantages of this invention will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawing figures and the written description.